A conventional tuner circuit comprises a UHF mixer for generating a UHF intermediate frequency signal by combining a UHF RF (radio frequency) signal and a local oscillation frequency signal, a VHF mixer for generating a VHF intermediate frequency signal by combining a VHF RF signal and a local oscillation frequency signal, and an IF amplifier for amplifying the UHF and VHF intermediate frequency signals, and a switching diode for connecting the UHF mixer to the VHF mixer.
In operation, the UHF RF signal is supplied to the UHF mixer along with the local oscillation frequency signal, so that the UHF intermediate frequency signal is generated in the UHF mixer. The UHF intermediate frequency signal is supplied through the switching diode to the VHF mixer which functions as an IF amplifier in the UHF operation. Thus, the UHF intermediate frequency signal is amplified in the UHF mixer, and the amplified UHF intermediate frequency signal is further amplified in the IF amplifier to provide a predetermined UHF intermediate signal which is supplied to a following stage in a television set or a video tape recorder system. In the VHF operation, on the other hand, the VHF RF signal and the local oscillation frequency signal are supplied to the VHF mixer, in which the VHF intermediate frequency signal is generated to be supplied to the IF amplifier. Thus, a predetermined VHF intermediate signal is supplied to a following stage in the television set or the video tape recorder system.
However, the conventional tuner circuit has disadvantages as follows.
(1) The control of a total gain in the UHF and VHF bands is difficult, because the VHF mixer is used as the IF amplifier for the UHF intermediate frequency signal.
(2) A change-over device such as the switching diode is necessary to connect the UHF mixer to the VHF mixer.
(3) The VHF RF signal tends to leak to the UHF mixer, and the UHF intermediate frequency signal tends to leak to an input terminal side for the VHF RF signal, because the VHF RF signal and the UHF intermediate frequency signal are applied to a common input terminal of the VHF mixer. Especially, the leakage of the VHF RF signal to the UHF mixer results in the deterioration of a noise factor.